Reheating for steam generators



Oct. 5, 1965 J. F. SCHONBERGER ETAL 3,209,731

REHEATING FOR STEAM GENERATORS 4 Sheets-Sheet 1 Filed Jan. 14, 1963 m M f a P w K 8 ..l\/. G wfw Y m w El\ E m w .w R E u 0 5 2 R N R E M 5 E 2 Y S w M s m a 0 0 m M 5 w 5 r 7 w r 0 j r m N m 4 r A 2 w a 7m 3 E W 4 5 4 .w l a Ii 0 5 I L, fill. I. 4 J 7 m A G I m l I m v M 3 m OK A l lmw/flwu T x 5 M L Em fmHn wU ATTORNEY 1965 J. F. SCHONBERGER ETAL 3,209,731

REHEATING FOR STEAM GENERATORS Filed Jan. 14, 1963 I 4 Sheets-Sheet 2 INVENTORS J Y E SCHO/VBE/PGf/Q 7 $B/A5 su e/v M444 x 7 ATTORNEY Oct. 5, 1965 .J. F. SCHONBERGER ETAL 3,209,731

REHEATING FOR STEAM GENERATORS Filed Jan. 14, 1963 4 Sheets-Sheet Z5 INVENTORS 7% Y F. JCHO/VSEEGER TOE/A5 J'TEAN BY ATTORNEY Oct. 5, 1965 J. F. SCHONBERGER ETAL 3,209,731

REHEATING' FOR STEAM GENERATORS Filed Jan. 14, 1965 4 Sheets-Sheet 4 INVENTORS JAY FJCHO/VEE/PGER TOS/flS STE/9N Mad/M 75mm? ATTORNEY United States Patent 3,209,731 REHEATING FOR STEAM GENERATORS Jay F. Schonberger, Totawa, and Tobias Stern, White Meadow Lake, N.J., assignors to Foster Wheeler Corporation, New York, N.Y., a corporation of New York Filed Jan. 14, 1963, Ser. No. 251,099 15 Claims. (Ci. 122459) This invention relates to a reheat cycle for steam generators, and in particular to a method and apparatus for reheating or resuperheating steam using reheaters of the shell and tube type.

In a conventional steam generator-turbine cycle, the working fluid is expanded in the first stage or high pressure turbine adiabatically, i.e. at constant entropy, from an initial temperature and pressure to a final or exhaust pressure, with a resulting decrease in the total heat available in the fluid. In this expansion, although it is customary to expand the working fluid only to a point above saturation temperature for the fluid at the pressure of the fluid at the turbine outlet, certain cycles may require that the steam be expanded to a point where at least a portion of it condenses. For instance, present turbine design may permit a moisture content of up to 10% in the exhaust flow.

In reheating the vapor-liquid mixture for expansion in subsequent turbine stages, the fluid will require the addition of heat at constant temperature until the liquid phase is vaporized. Then further heat is added at constant pressure to superheat the working fluid, although usually only to a point near but less than the original temperature of the working fluid. The fluid passes to the second stage or reheat turbine and from there to exhaust or a low pres sure turbine.

The use of reheat makes it possible to attain high thermodynamic efliciencies accompanying high initial pressures without the need for resorting to high initial steam temperatures, and without the neutralizing effect of high moisture in the later stages of the turbine. In this latter respect, a reduction in exhaust moisture means an improvement in stage efiiciency for this section of the turbine, and also reduces erosion on the low pressure turbine blades.

However, a chief objection to the use of reheat in the past has been the initial costs involved which sometimes overbalance the gain in fuel costs. By initial costs, it is meant greater unit length, higher costs of turbine and station piping and additional costs of reheater equipment. Because of these initial costs, the reheat cycle has often proved economical only on larger units where fuel costs and load factors are high.

In marine applications. the above disadvantages become even more significant, but if fuel costs continue to increase, it is apparent that the reheat cycle will be justified for smaller and smaller units.

One particular disadvantage in conventional reheat cycles for resuperheating a vapor-liquid mixture is that the heat-transfer to vaporize the mixture at constant temperature is dependent on a heat-transfer coefficient which is substantially that of the vapor and much less than the heat transfer coeflicient for the liquid. Therefore, the heat transfer surface required to vaporize the liquid of the vapor-liquid mixture will be much greater than that which would be required to vaporize a pure liquid stream of the same mass or flow rate.

These and other disadvantages are overcome in accordance with the invention, an object of which is to obtain reheat at a minimum initial equipment cost and with a minimum space requirement. It is a particular object of the invention to accomplish this for small steam generating units, particularly of the type used for marine installations.

More specifically, the invention will be useful with high pressure nuclear steam generating units in which the high pressure turbine inlets can be provided only with saturated or slightly superheated steam, the exhaust steam from the high pressure turbine therefor having a moisture content of from 8 to 10%. In subsequent turbine stages, the pressure is reduced to a point where superheated steam can be provided at the turbine inlet points.

In accordance with the invention, there is provided in a reheat cycle or process for a vapor-liquid turbine exhaust mixture which contains from 8 to 10% liquid, the steps of separating the mixture in a primary separation or drying zone into a first stream containing a substantially liquid component and a second stream containing a substantially vapor component; passing said first streamliquid component through a heating zone to vaporize at least a portion thereof; combining this vaporized portion with said second stream-vapor component; and resuperheating this combined vaporized portion and second stream. In an embodiment in accordance with the invention, the vapor-liquid mixture flowing from the heating zone is subjected to a coarse separation in a supplemental separation zone of the reheat cycle so that only the vaporized portion of the first stream is combined with the second stream-vapor component of the turbine exhaust mixture. The liquid from the supplemental separation zone is recycled to the heating zone. Preferably the vapor component of the coarse or supplemental separation is combined with the turbine exhaust mixture prior to the mixture passing through the primary separation or drying zone.

By separating from the turbine exhaust mixture the liquid component and heating this component in a separate boiling unit, where the heat transfer coetficient is that of the liquid and much higher than in the transfer of heat to a vapor and liquid mixture, the heat transfer area required is substantially reduced. It is estimated that for a steam and water mixture of from 8 to 10%, in ac cordance with the invention, the heat transfer area required for boiling the water in accordance with the invention will be from a quarter to a half of that required in conventional units where the full stream and water mixture is passed through the vaporizing section. With such a reduction in heat exchange surface, it is apparent that a substantial stride is made towards reducing initial equipment costs for the reheat cycle and towards justifying use of the reheat cycle for small vapor generating units, particularly those for marine applications.

It is well known to include in a vapor generating and superheat cycle means for vapor-liquid separation upstream of the superheater to remove solid material and impurities from the water, since these solids and impurities will be detrimental in the superheating section. However, the steam and water mixture exiting from a high pressure turbine is pure and uncontaminated, and within the knowledge of the applicant, it has never been proposed to use vapor-liquid separation in connection with reheat of a turbine exhaust fluid.

It has been proposed in connection with the reheat of steam from a high pressure turbine to separate out the moisture from the steam. However, in this instance, the separated moisture is fed to a system deaerator and back into the main generator.

The present invention constitutes an improvement over this proposal in that the heat in the moisture exiting from the high pressure turbine is made available to the intermediate and low pressure turbines.

An aspect of the invention resides in the apparatus by which reheat in accordance with the invention is accomplished. It must be recalled that a principal object of the invention is the conservation of space and reduction of initial equipment costs. Towards these objects, the

reheater comprises a single shell within which are disposed separate vapor generating and vapor resuperheating tube bundle surfaces separated by a drying region or zone, and means for introducing the vapor-liquid exhaust mixture to the drying region, the generating and resuperheating surfaces being disposed so that drains from the drying region pass to the generating surface and vapor from the drying region passes to the resuperheating surface. The geometry of the generating surface is that of a natural circulation generator so that the vapor generated flows upwardly for admixing with the vapor-liquid turbine exhaust mixture. Preferably, before admixing, means are provided for subjecting the upward flow to a coarse vapor-liquid separation so that only relatively dry vapor is admixed with the turbine exhaust flow, separated liquid being recycled to the vapor generating area. This prevents overloading of the primary drying region.

In an embodiment of the invention, a single tube bundle within the shell provides the vapor generating and resupcrheating surface required. The bundle is of the U-tube type and is baffied sealing off the upper section thereof, this section being used as a boiler while the lower section is used as a resuperheater. The drying region is disposed between the sealed off upper section and the resuperheating area in a manner so that drains from a vapor-liquid turbine exhaust mixture flow into the generating area and the vapor stream flows to the resuperheating area.

The invention and advantages thereof will become apparent upon consideration of the following detailed description and accompanying drawings, in which:

FIGURE 1 is a flow diagram illustrating the concepts of the invention as applied to a steam generating, turbine expansion and superheat cycle;

FIGURE 2 is a schematic flow diagram illustrating the manner in which apparatus in accordance with concepts of the invention may be applied to the cycle of FIG. 1;

FIGURE 3 is a schematic section elevation view of a resuperheat arrangement in accordance with the invention;

FIGURE 4 is a section view taken along line 44 of FIG. 3;

FIGURES 5-8 illustrate schematically section elevation and plan cross-section views of resuperheat arrangements in accordance with embodiments of the invention;

Referring to FIG. 1, the resuperheat cycle in accordance with the invention involves the steps of passing a steam and water mixture, which has been generated in a boiler 12 and adiabatically expanded in a high pressure turbine 14, to a separation or drying zone 16 where the mixture is separated into a vapor stream 18 and a liquid or water stream 20. It is apparent that the steam and water mixture containing approximately 8 to water, following high pressure turbine expansion and without separation or drying, will at a high velocity cause erosion in the low pressure turbine 22 whereas a correspondingly high steam velocity would be safe. Further, the entrained water in the turbine exhaust flow will fail to wet or will only partially wet the heat transfer surface in resuperheater 24, so that water in the exhaust flow will be vaporized at a steam overall heat transfer coefiicient rather than at the higher boiling overall heat transfer coeflicient.

Accordingly, the separated liquid flow in stream is passed to a separate heat exchange or boiling surface 26, the vapor from the heat exchange surface being recycled to the main flow through line 28. This combined main flow is then subjected to drying in zone 16, the resultant combined vapor streams being superheated in heat exchanger 24 for turbine 22.

In this way, the heat content in the liquid component of the exhaust mixture is conserved or retained and used for subsequent turbine expansion, while at the same time, a reduction in equipment costs and heat transfer surface is realized. In this latter respect, it is apparent that by vaporizing the liquid drains or liquid component of the exhaust mixture in a separate heat exchange area, the

boiling overall heat transfer coefiicient, which is higher, is utilized with a consequent reduction in the heating surface. This reduction may be a half /2) to three quarters A) of the surface area involved.

FIG. 2 illustrates the manner in which this is accomplished. The vapor generating and resuperheating surfaces are contained within a single shell 30, which IS divided into a boiling section 32, a resuperheating section 34, and a primary separation or drying zone 36. The exhaust flow from turbine 14 is passed to the separat on or drying zone 36, defined by a batlle 38 and comprising driers 4t), drains from the latter being passed along line 42 to the vapor generating or boiling section 32. The vapor flow from the driers 40 is passed by line 44 to the resuperheating section 34, and from there by line 46 to turbine 22 and subsequent exhaust.

A vapor-liquid flow generated in the boiler 32 is subjected to separation in area 46, drains from the separation being recycled and combined through line 48 with the liquid component from the drying section 40, vapor from the flow (line 50) being admixed with the turbine exhaust mixture passing to the drying section.

Details of the unit in accordance with the invention are illustrated in FIGS. 3 and 4. The apparatus comprises a substantially vertical shell 30 in which the resuperheating section 34 and steam generating section 32 are disposed. The unit is of the shell and tube type having a U-shaped tube bundle 52 arranged in the lower part of the shell, the tubes of the tube bundle being in communication with inlet and outlet chambers 54 and 56 for the heating fiuid. At the upper end of the shell, in a portion of enlarged diameter, a cylindrical baffle or sheet metal drum 58 is disposed defining a large pressure chamber or area 60, the chamber also encasing the upper bend portion 62 of the tube bundle 52 and sealing the bend portion from the remainder and lower part of the unit. The upper part of the chamber is provided with an inlet passage 64 for the steam and water mixture flowing from the high pressure turbine, and outlet passages 66 leading to the inside of the shell 30. Driers 68, which may be of any conventional type, are disposed at the outlets 66, so that the steam and water mixture entering the enclosed area 60, through passageway 64, flows through the driers 68, where the water and steam are separated, and through outlet passages 66 to the interior of the shell 30.

The separated water drains downwardly in tubes 70 into a water space 72, in which the tube bundle portion 62, functioning as the heating tubes of a natural circulation unit, is immersed. The bundle is partly encased with a dome-shaped baffie 74 having a riser 76 at its upper end leading to separation means 78. The separator 78 may be of the centrifugal type, water separated from the upflowing mixture passing downwardly into the water space 72 of the boiling or steam generating section 32, and steam flowing upwardly mixing with the steam and water turbine exhaust mixture for flow through the driers 68.

The dried steam passing through outlets 66 into the shell area flows downwardly through an annular space 80 defined by the shell 30 and the cylindrical baffle 58 into the superheating section 34 of the unit, the latter comprising the straight portions 82 of the tube bundle. The steam flows downwardly around baffles 84 and through outlet 86 to the low pressure turbine (not shown).

In the embodiment illustrated in FIGS. 5 and 6, the superheating and boiling sections 88 and 90 are separated by a bafile plate 92, the superheating section 88 occupying the upper part of the pressure shell 94 and the boiling section 90 occupying the lower portion of the shell. The boiling section comprises steam generating tubes 96 of the U-tube type in communication with inlet and outlet means 93 and 100 for the heating fluid. The U-tubes are encased by a shroud 102 disposed within a liquid space 104 of the boiler, the shroud defining with the shell an annular downcomer passageway 106 through which the liquid flows. The steam and water mixture flows upwardly within the shroud by natural circulation through a riser S and separators 110. The separated liquid in the mixture is recycled to the liquid space and the separated vapor passes upwardly through a vapor space 112 and driers 114. Also passing through the driers is the vapor-liquid exhaust flow admitted to the space 112 through shell inlet 116, the vapor from the driers flowing upwardly into the upper superheating section 88 and the liquid flowing downwardly into downcomers 113 to the liquid space 104 and the boiling section 90. The vapor to be superheated flows upwardly past baffles 120 and superheating tubes 122, extending between inlet and outlet headers 124 and 126, and through outlet means 128.

The embodiment illustrated in FIG. 7 involves the use of multiple tube bundles 130 and 132, within shell 133, the bundle 130 constiuting the superheating section of the reheater and being of the U-tube type, and the bundle 132 constituting the boiler section of the reheater and being of the L-tube type. The latter are disposed within the shell 133 in an annular chamber 134, defined on the sides and top by baffles 136, 138 and 1430, about the U- tube bundle or superheating section.

The heating fluid flows inwardly through inlet chamber 142, through the U-tube bundle or superheating section 130 into chamber 14-4, and then, simultaneously, through outlet 145 and through the L-tube bundle of the boiling section 132. The vapor-liquid turbine exhaust mixture flows inwardly through inlet 148 in the upper end of the shell 133 and through driers 150, separated liquid flowing downwardly through the annular space or downcomer 152 defined by baifle 136 and the shell 133 into the chamber 134 encasing the L-tube bundle boiling section 132. The steam and water mixture generated in the boiling region is passed through separators 154, the vapor from the separators being admixed with the vapor-liquid turbine exhaust mixture from inlet 148, and the liquid being recycled through the downcomer 152. Vapor passing through the driers 150 enters the collecting region 155 and flows downwardly through central passageway 156 past the U-tube bundle superheating section 130, within bafile 138, and through the superheater outlet nozzle 158. 1" he baflle 138 is in the form of an annular shroud formed to enclose the U-tube bundle 136 except for inlet opening and outlet nozzle (156 and 158 respectively), and further formed to separate the superheating section from the boiling section.

Although the method and apparatus of the invention have been described with respect to specific method steps and structural arrangements, it is understood that many embodiments will be known to those skilled in the art and within the spirit and scope of the invention as defined in the following claims.

What is claimed is:

1. A method for resuperheating a steam and water turbine exhaust mixture in a steam generator turbine expansion and reheat cycle wherein the mixture containing 8 to 10% water flows in a main flow path between successive turbines, comprising the steps of separating the mixture in a drying zone into a first stream containing substantially water and a second stream containing substantially steam, passing said first water stream through a heating zone, vaporizing at least a portion thereof, dividing said partially vaporized stream in a supplemental separation zone into a water portion and a steam portion, recycling said water portion to said heating zone and combining it with said first water stream, combining said steam portion directly with said exhaust mixture second stream and passing said combined steam portion and sec- 0nd stream through a superheating zone for resuperheat thereof.

2. A method according to claim 1 wherein said exhaust second stream is combined with said vaporized steam portion upstream of said drying zone.

3. A method according to claim 2 wherein said heating zone is sized for a water content of, 8 to 10% of the turbine-exhaust steam and water mixture, said sizing being a quarter to a half of that required to vaporize the Water when in the mixture state.

4, A method according to claim 2 wherein the steps between drying and combining the first stream vaporized portion with the turbine-exhaust mixture are eflected by natural circulation.

5. Apparatus for resuperheating a vapor and liquid exhaust mixture in a vapor generator-turbine expansion cycle comprising an upright elongated shell, means sealing 01? said shell into a boiling section and a superheating section, the boiling section having an upper vapor space and a lower liquid space, means for introducing a vapor and liquid mixture into said boiling section vapor space, an outlet in said boiling section for the flow of vapor from said vapor space to said superheating section, drier means disposed in said outlet means whereby said vapor and liquid mixture passes therethrough, said drier means being arranged such that the liquid separated from the mixture flows downwardly by natural circulation to the boiler section liquid space and vapor flows upwardly therethrough to the superheating section, the geometry of the boiling section being that of a natural circulation generator so that the vapor generated flows upwardly to said vapor space for admixing with the vapor-liquid turbine exhaust mixture.

6. Apparatus according to claim 5 wherein said boiling section is disposed above said superheating section, and a tube bundle of the U-tube type provides the heating surface for both said sections, said first mentioned means sealing-off the upper bend-area of the bundle for said boiling section, said means further defining with the shell an annular downcomer area by which vapor flowing through the drying region flows downwardly into the superheating section.

7. Apparatus according to claim 5 and further including a riser disposed above said boiling section for the upward flow of a vapor-liquid mixture therein, and vapor liquid separating means connected to said riser into which said mixture flows, said separating means being arranged whereby vapor therefrom flows to said dryer means and liquid flows downwardly recycling to said boiling section.

8. Apparatus for reheating a vapor-liquid turbine ex haust mixture in a steam-reheat cycle comprising an upright elongated shell, plate means dividing said shell into an upper superheating section and a lower boiling section, said boiling section having a vapor space and a liquid space and comprising a plurality of steam generating tubes of the U-tube type in said liquid space through which a heating fluid flows, shroud means encasing said U-tubes and defining a vapor-liquid generating chamber, riser means above said chamber, separating means in communication with said riser means, said separating means being arranged whereby vapor flows upwardly into said vapor space and liquid is recycled to said liquid space and said vapor-liquid generation chamber, a drier region disposed between said boiling section and said superheating section and arranged for the flow of vapor to the superheating section and the flow of liquid to the liquid space of the boiling section, means for introducing a vapor-liquid turbine exhaust mixture to said boiling section vapor space for flow through said drier region, the upflowing vapor from said boiling section admixing with said vapor-liquid exhaust mixture, said superheating section comprising means for the flow of dried vapor therethrough including outlet means at the upper end thereof.

9. Apparatus for reheating a vapor-liquid turbine exhaust mixture in a steam-reheat cycle comprising an upright elongated shell; means sealing oflf said shell into a boiling section and a superheating section; said superheating section comprising at least one tube bundle of the U-tube type and the boiling section comprising at least one tube bundle of the L-tube type, arranged whereby the U-tube bundle is disposed in the center portion of the shell and the L-tube bundle is disposed in an annular space around the U-tube bundle; means for passing a heating fluid through said tube bundles; a drying region separating said boiling section from said superheating section; means for introducing a vapor-liquid turbine exhaust mixture into said drying region; means for admixing the vapor-liquid flow generated in said boiling section with the vapor-liquid turbine exhaust mixture, said drying region being arranged whereby the liquid separated therein drains downwardly into said boiling section, the geometry of said boiling section being of the natural circulation type.

10. A method for resuperheating a vapor and liquid exhaust mixture in a vapor generator turbine expansion cycle wherein the mixture flows in a main flow path between successive turbines comprising the steps of separating a liquid component from the main flow path such that a substantially vapor component remains;

heating said liquid component to vaporize at least a portion thereof;

resuperheating said vapor component;

reintroducing the vaporized portion of said liquid component directly to said main flow path such that it is resuperheated with the vapor component.

11. A method according to claim 10 wherein the vaporized portion of the liquid component is reintroduced to the main flow path by combining it with the mixture prior to the separating said liquid component from the main flow path.

12. A method according to claim 10 including the step of subjecting the vaporized portion of the liquid component to a supplemental separation to remove liquid entrained therein.

13. A method according to claim 12 wherein the liquid removed by said step of supplemental separation is recycled for heating with additional liquid component separated from the main flow path.

14. A method according to claim 10 wherein the liquid content of the vapor and liquid exhaust mixture is from 8 to 10%.

15. Apparatus for resuperheating a vapor and liquid mixture in a vapor generator-turbine expansion cycle comprising an upright elongated shell;

means sealing-off said shell into a lower boiling section and an upper superheating section, the boiling section having an upper vapor space and a lower liquid space;

separate tube bundles in each said section adapted to convey a heating fluid for the boiling section and the superheating section;

outlet means in said boiling section for the flow of vapor to said superheating section;

dryer means disposed in said outlet means whereby the flow from said boiling section to said superheating section passes therethrough;

means for introducing a turbine exhaust mixture into the vapor space of said boiling section, the dryer means being arranged whereby liquid therefrom drains to the liquid space of the boiling section and separated vapor flows to said superheating section,

the boiling section being a natural circulation generator by which the vapor generated therein flows upwardly into said vapor space for admixture with said exhaust mixture.

References Cited by the Examiner UNITED STATES PATENTS 2,336,832 12/43 Badenhausen 122-34 2,739,575 3/56 Byerley 122-34 2,862,479 12/58 Blaser et al 122-34 2,922,404 l/ Kopp et al. 122-34 2,959,014 11/60 Artsay 122-479 X 2,995,34-3 8/61 Gardner et al. 122-483 X 3,071,119 1/63 Ammon et al. 122-34 3,076,444 2/63 Bell 122-34 3,129,564 4/64 Brunner 122-406 X 3,141,445 7/64 Bell 122-34 FOREIGN PATENTS 841,656 7/60 Great Britain. 876,308 8/61 Great Britain.

FREDERICK L. MATTESON, JR., Primary Examiner.

PERCY L. PATRICK, Examiner. 

10. A METHOD FOR RESUPERHEATING A VAPOR AND LIQUID EXHAUST MIXTURE IN A VAPOR GENERATOR TURBINE EXPANSION CYCLE WHEREIN THE MIXTURE FLOWS IN A MAIN FLOW PATH BETWEEN SUCCESSIVE TURBINES COMPRISING THE STEPS OF SEPARATING A LIQUID COMPONENT FROM THE MAIN FLOW PATH SUCH THAT A SUBSTANTIALLY VAPOR COMPONENT REMAINS; HEATING SAID LIQUID COMPONENT TO VAPORIZE AT LEAST A PORTION THEREOF; RESUPERHEATING SAID VAPOR COMPONENT; REINTRODUCING THE VAPORIZED PORTION OF SAID LIQUID COMPONENT DIRECTLY TO SAID MAIN FLOW PATH SUCH THAT IT IS RESUPERHEATED WITH THE VAPOR COMPONENT. 